Internal combustion engine exhaust emissions, and especially diesel engine exhaust emissions, have recently come under scrutiny with the advent of stricter regulations, both in the U.S. and abroad. While diesel engines are known to be more economical to run than spark-ignited engines, diesel engines inherently suffer disadvantages in the area of emissions. For example, in a diesel engine, fuel is injected during the compression stroke, as opposed to during the intake stroke in a spark-ignited engine. As a result, a diesel engine has less time to thoroughly mix the air and fuel before ignition occurs. The consequence is that diesel engine exhaust contains incompletely burned fuel known as particulate matter, or “soot”.
It is known to use catalytic particulate filters which physically trap the soot particulates. However, catalytic diesel particulate filters (CDPF) progressively load up with accumulated soot and therefore must be repeatedly regenerated by catalytically oxidizing the trapped particulates, typically on a fixed schedule and by fuel enrichment of the exhaust stream entering the CDPF.
Typically, prior art regeneration systems are temperature based with the primary protection strategy being limitation of the quantity of soot allowed to accumulate. As shown below, this serves to under-utilize the filter capacity and results in a penalty in fuel economy.
A currently challenging durability issue in the CDPF art is cracking or melting of a CDPF substrate due to large temperature excursions within the bed of the filter during regeneration. These temperature excursions are caused by the exothermic reaction of carbon and oxygen when the soot loading exceeds approximately 5 grams per liter of CDPF substrate and the flow rate of cooling exhaust through the CDPF is reduced by idle or low-load engine operating conditions. Under these conditions, the exhaust contains a high percentage of oxygen (18% or more), thus fueling a very rapid combustion of the soot. A low total flow rate reduces convective cooling of the hot substrate. This combination of events (rapid combustion and low cooling) can result in excessive filter temperature and/or temperature gradients, resulting in substrate failure.
A factor only recently recognized in CDPF regeneration is the relative combustibility of wet soot versus dry soot, both of which can be present in a CDPF. So-called “wet” soot is soot impregnated with unburned hydrocarbon fuel residues and is produced during conditions of low engine combustion efficiency. Wet soot is now known to be much more volatile and to burn much hotter than dry soot during catalytic regeneration. A high level of wet soot in a CDPF can lead to an uncontrolled combustion event that can damage or completely destroy a CDPF. Thus, it is important to be able to monitor the percentage of wet soot accumulated in a CDPF as well as the overall amount of soot.
U.S. Pat. No. 6,735,941 B2 discloses a method for calculating the total soot mass accumulated in a CDPF by measuring differential pressure across the CDPF. This method does not recognize the functional (combustional) difference between wet soot and dry soot; does not determine the percentage of total soot that is wet soot; and does not provide a strategy for burning off the wet soot in a controlled manner before completing oxidation of the dry soot, to protect against thermal damage to a CDPF.
U.S. Pat. No. 7,231,761 discloses a method for regeneration of a CDPF wherein a post-combustion injection quantity of fuel required during regeneration is determined by means of measured soot mass accumulated with the CDPF. (As used herein, the term “post-combustion” refers to engine combustion in the engine cylinders.) During regeneration, this post-combustion fuel injection value is corrected by application of a correction factor determined from the deviation between target temperature and actual temperature within the CDPF in an attempt to prevent an uncontrolled burnout from occurring. This method also does not provide a strategy for burning off the wet soot in a controlled manner before completing oxidation of the dry soot as is highly desirable to protect against thermal damage to a CDPF.
What is needed in the art is a method for calculating the wet soot fractional percentage of a soot load in a CDPF; determining a “combustibility value” for the overall soot content; and selecting a thermal management strategy to be used during regeneration that controls the temperature increase rate and, hence, the thermal gradient of the CDPF monolith to burn off the volatile compounds in the wet soot in a controlled manner before completing oxidation of the dry soot.
It is a principal object of the present invention to prevent damage to a CDPF substrate by overheating during regeneration thereof.